[flang] Check functions that implement defined operators (flang-compiler/f18#846)

Section 15.4.3.4.2 specifies restrictions on functions that may be used
to implement an "extended-intrinsic-op". These checkw are implemented in
`CheckHelper::CheckDefinedOperator`.

Move `IsIntrinsicRelational` et al. to `semantics/tools.h` so that
the same logic is used to check both dummy and actual arguments.

Fix up tests that had errors that are now detected.

Original-commit: flang-compiler/f18@b900762eed5883bfbc31349847381098de1e7b6a
Reviewed-on: https://github.com/flang-compiler/f18/pull/846